JP5331950B2 - Optical fiber laser light source - Google Patents

Description

  The present invention relates to an optical fiber amplifying module that amplifies input light with an amplifying optical fiber.

  An optical fiber laser light source is known as a highly reliable and small-sized light source, and particularly has a good beam quality because it has a waveguide structure. For this reason, optical fiber laser light sources are widely used in various applications such as processing, medical treatment, measurement, and optical communication in the electronic / mechanical field.

In particular, an optical fiber laser light source of MOPA (Master Oscillation Power Amplifier) configuration that amplifies pulse seed light output from a seed light source by an optical fiber amplification module is a fiber in comparison with a light source of Q switch configuration that is currently popular. No broadening of pulse width due to propagation delay due to length is observed. That is, the optical fiber amplification module is advantageous for shortening the pulse and increasing the repetition frequency, and is expected to be widely used for fine processing and measurement. However, when a semiconductor laser light source capable of high-speed modulation is used as a seed light source and applied to an application requiring high output such as processing, the optical fiber amplification module is required to have a high gain of 50 dB or more. Since the amplification action in the optical fiber amplification module is usually realized by an amplification optical fiber to which a rare earth element or the like is added, in principle, a high gain is easily realized.
Japanese Patent No. 3306700 FD Teodoro, et al, "High-power pulsed fiber source at 1567nm", Proc. Photonic WEST2005

  The inventor discovered the following problems as a result of examining the prior art in detail. That is, in the optical fiber amplifying module, when the mixed component ratio of the amplified spontaneous emission light (ASE) increases and the extinction ratio of the output pulse train deteriorates, the output of the optical fiber amplifying module is saturated with the ASE light. Gain is difficult to obtain. Therefore, it is desirable to dispose a dielectric multilayer bandpass filter on the propagation path of the amplified light and select only the amplified light by the dielectric multilayer bandpass filter. Note that the pulse peak is higher in laser processing applications and the like than in applications where the signal light level is low, such as for optical communication, and therefore there is a high possibility of destroying the dielectric multilayer film. For example, Non-Patent Document 1 proposes a method in which an optical fiber Bragg grating and an optical circulator are combined.

  As described in Patent Document 1, it is conceivable to prevent destruction of the optical filter by controlling the gain or optical power level up to the upstream portion of the optical filter in the optical fiber amplification module. However, in a processing laser or the like, it is difficult to detect a pulse peak when the duty ratio is as low as 1/100 to 1/10000. Therefore, active control as described in Patent Document 1 is time constant or the like. There is a high risk of malfunction due to restrictions.

  On the other hand, the high gain due to the amplification optical fiber is excessively high depending on the operating state, and oscillation that causes mode competition by itself occurs. In this case, there is a concern that an optical component constituting the optical fiber amplification module may be destroyed by an output such as an optical surge. In particular, in the clad excitation method widely used for realizing high output, there are many cases where the excitation light and the light to be amplified are not separated. In this case, there is a risk that an optical surge due to the amplified light or ASE light that should propagate through the core is coupled to the propagation mode of the inner cladding through which the excitation light propagates, leading to the destruction of the excitation light source. Therefore, it is desirable that the gain of the clad excited portion is not so high. Generally, about 20 dB is said to be an upper limit guide.

  The present invention has been made to solve the above-described problems, and an optical fiber amplifying module having a structure for stably realizing high gain even when light with a low duty ratio is amplified. The purpose is to provide.

  An optical fiber amplifying module according to the present invention outputs an input end for inputting light having a duty ratio of 1% or less and amplified light obtained by amplifying input light taken in through the input end A first control means for controlling an amplification gain so as to avoid destruction of the optical filter, an optical filter, an optical filter, and an output end, a plurality of amplification optical fibers (at least three amplification optical fibers), At least. The duty ratio is defined by “pulse width / pulse period”.

  The plurality of amplification optical fibers are arranged in a column along the light propagation path from the input end to the output end, and each amplifies the input light. The optical filter is an optical component for selectively allowing the input light taken in via the input end to pass through. The optical filter includes, among the plurality of amplification optical fibers, the first-stage amplification optical fiber through which the input light that has passed through the input end first passes, and the amplification light output from the first-stage amplification optical fiber first. Between the second-stage amplification optical fibers passing through In particular, the first control means sets the gain for the input light in the first-stage amplification optical fiber to be equal to or less than the first predetermined value.

  In the optical fiber amplifying module according to the present invention, the first control means may be configured with only an optical passive component or may have any feedback configuration. For example, the first control means supplies the first-stage amplification optical fiber to the first-stage amplification optical fiber in a state where the absorption length product of the first-stage amplification optical fiber is optimized to a specific value in a temperature-controlled environment. The excitation light source that supplies the excitation light is controlled so that the power of the excitation light is kept constant. At this time, the ambient temperature around the amplification optical fiber is preferably controlled so as to be within a specific range. For such temperature control, a method of performing feedback control on the Peltier element while performing temperature detection with a thermistor is conceivable. However, the amplification light is formed on a material having good thermal conductivity that can substantially serve as a heat sink. A system in which a fiber is mounted and the heat sink is forcibly cooled by a fan may be used. In this way, in the configuration in which the gain in the first-stage amplification optical fiber is suppressed by controlling the absorption-length product and the pumping light power of the first-stage amplification optical fiber, the absorption-length product of the first-stage amplification optical fiber is controlled. The first control means can also be realized by setting the pump light power and the pump light power to optimum values in advance.

  In the optical fiber amplifying module according to the present invention, the first control means can be realized by a feedback configuration. In this case, the first control means can control the ASE light output from the first-stage amplification optical fiber. Among them, a configuration is provided for monitoring wavelength components other than the input light and controlling the power of the pumping light supplied to the first-stage amplification optical fiber according to the monitored value. Further, the first control means transmits both input light and selectively reflects specific wavelength components around the input light wavelength, which are arranged at both the light input end and the light output end of the first-stage amplification optical fiber. Two reflecting means for the purpose may be included. The predetermined value may be a specific value or a numerical value range. Further, the control by the first control means may be always performed, or the control may be performed only when a predetermined value is exceeded.

  In the optical fiber amplifying module according to the present invention, the gain with respect to the input light in the final-stage amplifying optical fiber arranged at the position closest to the output end among the plurality of amplifying optical fibers is equal to or less than the second predetermined value. You may provide the 2nd control means to set as follows. The second predetermined value is generally set to about 20 dB as described above, and is usually smaller than the first predetermined value. Specifically, the two reflecting means for selectively transmitting a specific wavelength component different from the input light while transmitting the input light and arranged so as to sandwich the optical fiber for amplification at the final stage are used. Two control means are feasible. At this time, of the two reflecting means, the reflecting means located on the downstream side when viewed from the propagation direction of the input light is used for the amplification of the final stage through which the amplified light output from the amplification optical fiber of the final stage propagates. It arrange | positions in the space between an optical fiber and an output end. The specific wavelength component that is selectively reflected is preferably shorter than any wavelength component included in the input light (amplified light).

  Furthermore, an optical fiber amplifying module according to the present invention has an input end for inputting light having a duty ratio of 1% or less, and amplified light obtained by amplifying input light taken in through the input end , A plurality of amplification optical fibers (at least three amplification optical fibers) to which yttrium (Yb) is added, and an optical filter. In this case, the plurality of amplification optical fibers are arranged in a column along the light propagation path from the input end to the output end. Further, the optical filter selectively passes the input light taken in through the input end, so that the first-stage amplification light through which the input light that has passed through the input end first passes among the plurality of amplification optical fibers. It is arranged between the fiber and the second-stage amplification optical fiber through which the amplified light output from the first-stage amplification optical fiber first passes. In particular, in the optical fiber amplifying module having such a structure, the first-stage amplification optical fiber has an unsaturated absorption length product of 1000 dB or less with respect to light propagating in the first-stage amplification optical fiber. Is preferred.

  Each embodiment according to the present invention can be more fully understood from the following detailed description and the accompanying drawings. These embodiments are shown merely for illustrative purposes and should not be considered as limiting the invention.

  Further scope of applicability of the present invention will become apparent from the detailed description given below. However, the detailed description and specific examples, while indicating the preferred embodiment of the invention, are presented for purposes of illustration only and various modifications and improvements within the spirit and scope of the invention. Will be apparent to those skilled in the art from this detailed description.

  As described above, according to the present invention, even when light with a low duty ratio (light to be amplified) is amplified, high gain can be stably realized.

  Hereinafter, each embodiment of the optical fiber amplification module according to the present invention will be described in detail with reference to FIGS. In the description of the drawings, the same elements and the same parts are denoted by the same reference numerals, and redundant description is omitted.

(First embodiment)
FIG. 1 is a diagram showing a configuration of a first embodiment of an optical fiber amplifying module according to the present invention. The optical fiber amplification module 1 according to the first embodiment shown in FIG. 1 amplifies the light input to the input connector 11 and outputs the amplified light from the output collimator 12 as collimated light. The optical fiber amplification module 1 includes an optical isolator 21, an optical coupler 31, a YbDF 41, an optical isolator 22, a bandpass filter 50, an optical coupler 32, a YbDF 42, and an optical isolator arranged in order from the input connector 11 to the output collimator 12. 23, an optical fiber Bragg grating 60, a combiner 70, and a YbDF43. The optical fiber amplifying module 1 further includes a pumping light source 81 connected to the optical coupler 31, a polarization beam combiner 33 connected to the optical coupler 32, and pumping light sources 82 and 83, and a plurality of units connected to the combiner 70. The excitation light source 84 is also provided.

  Each of the YbDFs 41 to 43 is an amplification optical fiber (Yb-Doped Fiber) in which quartz glass is used as a host glass and Yb element is added as a photoactive substance to the optical waveguide region. In particular, the YbDF 43 provided in the last stage includes a core 43a (maximum refractive index n1) to which the light to be amplified propagates while adding the Yb element, as shown in the region (a) of FIG. A cladding region 43b surrounding 43a is provided. The cladding region 43b includes an inner cladding 43b1 (refractive index n2 (<n1)) through which pumping light from a plurality of subsequent pumping light sources 72 propagates, and an outer cladding 43b2 (refractive index n3 (<n2)) surrounding the inner cladding 43b1. It is composed of 2 is a refractive index profile 430 of the YbDF 43. In the refractive index profile 430, the region 431 is the refractive index of each part along the radial direction of the core 43a, and the region 432 is the inner cladding. The refractive index of each part along the radial direction of 43b1, and the region 433 indicate the refractive index of each part along the radial direction of the outer cladding 43b2.

  In the first embodiment, the gain in the first stage YbDF 41 is controlled to be equal to or lower than the first predetermined value by the first control means configured only by passive optical components. That is, in the first embodiment, the first control unit is realized by setting the absorption length product of the first stage YbDF 41 and the power of the pumping light output from the pumping light source 81 to optimum values in advance. The On the other hand, the gain in the final stage YbDF 43 is also controlled by the second control means so as to be equal to or lower than the second predetermined value. This second control means is realized by reflecting means arranged at both ends of the final stage YbDF43. The second predetermined value is generally set to about 20 dB as described above, and is usually smaller than the first predetermined value.

  The excitation light source 81 is a single mode excitation LD that outputs excitation light (975 nm wavelength band, power 300 mW class) to be supplied to the YbDF 41. The excitation light sources 82 and 83 are single mode excitation LDs that output excitation light (975 nm wavelength band, power 500 mW class) to be supplied to the YbDF 42. The plurality of excitation light sources 84 are multimode excitation LDs that output excitation light (915 nm wavelength band) to be supplied to the YbDF 43. Each of these YbDFs 41 to 43 can amplify light in the 1064 nm wavelength band.

  Each of the optical isolators 21 to 23 allows light to pass in the forward direction from the input connector 11 to the output collimator 12, but does not allow light to pass in the reverse direction. The optical coupler 31 outputs the pumping light that has arrived from the pumping light source 81 to the YbDF 41, and also outputs the input light (light to be amplified) that has arrived from the optical isolator 21 to the YbDF 41. The polarization beam combiner 33 combines the excitation light output from the excitation light sources 82 and 83 with polarization. Furthermore, the polarization beam combiner 33 outputs the excitation light synthesized in this way to the optical coupler 32. The optical coupler 32 outputs the excitation light reaching from the polarization beam combiner 33 to the YbDF 42 and also outputs the amplified light reaching from the optical filter (bandpass filter) 50 to the YbDF 42. The combiner 70 outputs the amplified light that has arrived from the optical fiber Bragg grating 60 to the YbDF 43, and also outputs the excitation light that has arrived from the plurality of excitation light sources 84 to the YbDF 43.

  The band pass filter 50 is disposed between the first stage YbDF 41 and the second stage YbDF 42, more specifically, between the optical isolator 22 and the optical coupler 32. The band-pass filter 50 selectively transmits the amplified light out of the light reaching from the optical isolator 22 to the optical coupler 32 while blocking other wavelength components. The band pass filter 50 includes, for example, a dielectric multilayer film. The optical fiber Bragg grating 60 is disposed between the optical isolator 23 and the combiner 70, and selectively reflects a specific wavelength component different from the light to be amplified while transmitting other wavelength components.

  FIG. 3 is a diagram for explaining the configuration of the output collimator 12 included in the optical fiber amplification module 1 according to the first embodiment. In FIG. 3, a plurality of parts constituting the output collimator 12 are shown in an exploded state. The output collimator 12 includes an optical fiber 120, an end cap 121, a lens 122, a reflective film 123, and an optical isolator 124. In the first embodiment, the second control means described above is constituted by the optical fiber Bragg grating 60 and the reflective film 123.

  One end of the optical fiber 120 is optically connected to the final stage YbDF 43, and an end cap 121 is provided at the other end of the optical fiber 120. The lens 122 collimates the light output from the end cap 121. The reflecting film 123 receives light collimated by the lens 122 and reflects the same wavelength component as the specific reflection wavelength of the optical fiber Bragg grating 60 while transmitting other wavelength components. The reflection film 123 is composed of, for example, a dielectric multilayer film. The optical isolator 124 allows the light reaching from the reflective film 123 to pass outside, but does not allow the light to pass in the reverse direction.

  The optical fiber Bragg grating 60 and the reflection film 123 provided at both ends of the final stage YbDF 43 are optical components that selectively reflect a specific wavelength component different from the light to be amplified and transmit other wavelength components. The optical resonator composed of the optical fiber Bragg grating 60 and the reflection film 123 controls the gain in the YbDF 43 to be a predetermined value or less. That is, the second control means in the first embodiment is constituted by the optical fiber Bragg grating 60 and the reflection film 123. The reflective film 123 located on the downstream side is provided in a space after the amplified light output from the YbDF 43 is output from the end cap 121. The specific wavelength component reflected by the optical fiber Bragg grating 60 reflecting film 123 is preferably shorter than the wavelength of the light to be amplified.

  The optical fiber amplification module 1 operates as follows. That is, the excitation light (975 nm wavelength band) output from the excitation light source 81 is supplied to the YbDF 41 from the forward direction via the optical coupler 31. Excitation light (975 nm wavelength band) output from the excitation light sources 82 and 83 is subjected to polarization synthesis by the polarization beam combiner 33. Then, the excitation light synthesized in this way is supplied to the YbDF 42 from the forward direction via the optical coupler 32. Excitation light (915 nm wavelength band) output from the plurality of excitation light sources 84 is supplied to the YbDF 43 from the forward direction via the combiner 70.

  Light to be amplified (wavelength 1064 nm) input to the input connector 11 is input to the YbDF 41 via the optical isolator 21 and the optical coupler 31, and is amplified in the YbDF 41. The light amplified in the YbDF 41 is input to the YbDF 42 through the optical isolator 22, the band pass filter 50, and the optical coupler 32, and is also amplified in the YbDF 42. The light amplified in the YbDF 42 is input to the YbDF 43 through the optical isolator 23, the optical fiber Bragg grating 60, and the combiner 70, and further amplified in the YbDF 43.

  The light amplified in the YbDF 43 is finally collimated by the output collimator 12 and output to the outside of the optical fiber amplification module 1. In the output collimator 12, the light output from the YbDF 43 is output to the space via the optical fiber 120 and the end cap 121. The output light is collimated by the lens 122, and then the collimated light is output to the outside of the optical fiber amplification module 1 through the reflection film 123 optical isolator 124.

  That is, in the optical fiber amplification module 1, the light to be amplified input via the input connector 11 is amplified by the YbDFs 41 to 43, and the amplified light is output from the output collimator 12 to the outside of the optical fiber amplification module 1 as collimated light. Is output. For example, a pulse-modulated YAG laser or LD is coupled to the input connector 11 as a seed light source, and the light output from the output collimator 12 is used for processing and measurement.

  FIG. 4 is a table summarizing the specifications of the YbDFs 41 to 43 as samples of the amplification optical fiber included in the optical fiber amplification module 1 according to the first embodiment. YbDF41 has an unsaturated absorption peak of 250 dB / m, a core diameter of 2.4 μm, and a cladding diameter of 125 μm. YbDF42 has an unsaturated absorption peak of 180 dB / m, a core diameter of 4.0 μm, and a cladding diameter of 125 μm. YbDF43 has a double clad structure, and has an unsaturated absorption peak of 9 dB / m, a core diameter of 15.0 μm, and a clad diameter (inner clad diameter) of 125 μm. Only YbDF43 has a low unsaturated absorption peak because it assumes cladding excitation instead of core excitation. The cladding diameter of YbDF43 indicates the inner cladding diameter. In the YbDF 43, a resin coating with a low refractive index (outer cladding 43b2) is provided outside the inner cladding 43b1, thereby enabling cladding excitation. The full width at half maximum of the bandpass filter 50 is 3 nm.

  When a cheap LD capable of high-speed modulation is applied as the seed light source, the current that can be high-speed modulated is about 100 mA. As a result, the pulse peak of the seed light output from the LD is often 20 to 30 mW at most. Further, in the processing laser, the pulse width is 10 ns to 100 ns, and the repetition frequency is mostly set in the range of 10 kHz to 100 kHz. For example, when the pulse width is 10 ns and the repetition frequency is 10 kHz, the duty ratio becomes the minimum value (1/10000). When the pulse width is 100 ns and the repetition frequency is 100 kHz, the duty ratio becomes the maximum value (1/100). The duty ratio is defined by “pulse width / pulse period”. When the peak value of the seed light source is 30 mW, the average light power of the seed light output from the seed light source ideally changes in the range of −25 dBm to −5 dBm.

  FIG. 5 and FIG. 6 are graphs showing longitudinal distributions of light levels in the optical fiber amplifying module 1 according to the first embodiment. FIG. 7 is a graph showing the longitudinal distribution of the light level in the optical fiber amplification module according to the comparative example. In the configuration of the optical fiber amplification module according to the comparative example, the bandpass filter 50 is removed from the configuration of the optical fiber amplification module 1 according to the first embodiment. Each of FIGS. 5 to 7 shows the level distribution (signal) of the light to be amplified, the level distribution of pumping light in the forward direction (Pump-f), the level distribution of pumping light in the reverse direction (Pump-b), and the forward direction. An ASE light level distribution (ASE-f) and a reverse ASE light level distribution (ASE-b) are shown. 5 and 7 illustrate a case where the average optical power of the amplified light input via the input connector 11 is −25 dBm. FIG. 6 shows a case where the average optical power of the amplified light input to the input connector 11 is −5 dBm.

  As shown in FIG. 5, in the optical fiber amplifying module 1 according to the first embodiment, when the average optical power of the amplified light input through the input connector 11 is −25 dBm, the output collimator 12 The average light power of the output light is 2 W, and the duty ratio is 1/10000. Therefore, the output pulse peak value in this case is 20 kW. On the other hand, as shown in FIG. 6, in the optical fiber amplifying module 1 according to the first embodiment, when the average optical power of the amplified light input via the input connector 11 is −5 dBm, the output collimator The average optical power of the light output from 12 exceeds 10 W, and the duty ratio is 1/100. Therefore, the output pulse peak value in this case is 1 kW. In any case, since the pulse energy exceeds 0.1 mJ, processing such as marking can be performed.

  On the other hand, as shown in FIG. 7, in the optical fiber amplification module according to the comparative example from which the bandpass filter 50 is removed, the average optical power of the amplified light input through the input connector 11 is − In the case of 25 dBm, the average optical power of the light output from the output collimator 12 is reduced by 7 dB. That is, the pulse peak power is only 4 kW, and the pulse energy is 0.04 mJ, so that the application is extremely limited.

  However, in general, a bandpass filter is composed of a dielectric multilayer film and has a very high possibility of being destroyed when pulse light having a high peak power is input. The maximum input power allowable value of a commercially available bandpass filter is about 0.5 W to 1 W. Therefore, it is desirable that the gain from the input connector 11 to the exit of the optical isolator 22 is always about 15 dB (= 1 W / 30 mW).

  For this purpose, it is conceivable to perform automatic gain constant control (AGC) for the first stage YbDF 41 as described in Patent Document 1. However, when a low duty ratio is assumed, it is difficult to detect only the peak power, and the time constant of feedback control to the control circuit becomes a problem, so that application of AGC is difficult. As a countermeasure, the excitation power, Yb concentration and length of YbDF41, and in some cases, the amplified signal are set so that a small signal gain state is obtained in the entire range (−25 dBm to −5 dBm) of the average optical power of the input amplified light. A method for optimizing the optical wavelength is conceivable. Specifically, the unsaturated absorption length peak of YbDF41 is desirably 1000 dB or less.

  In the case of an amplification optical fiber (EDF: Erbium-Doped Fiber) in which Er element is added to the optical waveguide region as a photoactive substance, the excitation life is as long as 10 ms. Therefore, as shown in FIG. 8, when the power of the input light to be amplified reaches 5 dBm, the saturation of the gain starts, and the magnitude of the gain that can pass through the dynamic range of −25 dBm to −5 dBm is Even the wavelength 1567 nm described in Non-Patent Document 2 varies greatly. On the other hand, in the case of YbDF, since the excitation lifetime is as short as about 1 ms, even if the power of the input amplified light reaches -5 dBm, it is possible to maintain the small signal gain state. FIG. 8 is a gain spectrum of EDF. FIG. 8 shows a gain spectrum (G (−25)) when the input level is −25 dB, a gain spectrum (G (−20)) when the input level is −20 dB, and an input level of −15 dB. Gain spectrum (G (−15)), gain spectrum (G (−10)) when the input level is −10 dB, and gain spectrum (G (−5)) when the input level is −5 dB, respectively. Has been.

  Region (a) in FIG. 9 is a gain spectrum of the first stage YbDF 41, and region (b) in FIG. 9 is an enlarged view of a partial region thereof. The gain at a wavelength of 1064 nm is 20 dB. From the input connector 11 to the optical isolator 22, two optical isolators and at least four fusion splice points are included. Here, the specification of the optical isolator insertion loss in the 1064 nm wavelength band is 2 dB / piece, and considering that the fusion loss is about 0.2 dB to 0.4 dB per location, the average of the input amplified light is considered. If the optical power is in the range of −25 dBm to −5 dBm, it can be seen that the gain during this period is maintained at about 15 dB. In addition, the gain spectrum (G (−25)) when the input level is −25 dB and the gain spectrum (G (−20) when the input level is −20 dB are included in the area (a) and the area (b) of FIG. 9. )), Gain spectrum (G (-15)) when the input level is -15 dB, gain spectrum (G (-10)) when the input level is -10 dB, and gain when the input level is -5 dB Each spectrum (G (-5)) is shown.

  The band-pass filter 50 is preferably inserted only immediately after the first stage YbDF 41. This is because there is a restriction due to the maximum input power of the bandpass filter 50 and it is rather advantageous for making the gain of the YbDF 43 clad pumped in the final stage constant. An example of the latter reason is shown in FIG.

  FIG. 10 is a gain spectrum of the final stage YbDF43. Since the band pass filter 50 is provided only between the first stage YbDF 41 and the second stage YbDF 42, the ASE light generated in the second stage YbDF 42 is injected into the final stage YbDF 43. As a result, as shown in FIGS. 5 and 6, when the average power of the input amplified light is −25 dBm, the power of the amplified light output from the second stage YbDF 42 is less than 20 dBm. On the other hand, since the power of the forward ASE light is close to 25 dBm, the average input power viewed from the YbDF 43 does not differ greatly regardless of whether the average input power is −25 dBm or −5 dBm. As a result, as shown in FIG. 10, the gain of the final stage YbDF 43 at a wavelength of 1064 nm is 17 dB to 18 dB, and does not exceed the above-mentioned 20 dB. Here, FIG. 10 shows a gain spectrum (G (−25)) when the input level is −25 dB, a gain spectrum (G (−20)) when the input level is −20 dB, and an input level of −15 dB. Gain spectrum (G (-15)), gain spectrum (G (-10)) when the input level is -10 dB, and gain spectrum (G (-5)) when the input level is -5 dB. It is shown. Further, FIG. 10 also shows the gain spectrum in the embodiment when the operation is abnormal (the state where the excitation light source is deteriorated).

  Note that the above description is limited to the case of normal operation. For example, when both or one of the excitation light sources 82 and 83 deteriorates and the output power from the excitation light sources 82 and 83 deteriorates to 100 mW, the gain at a wavelength of 1064 nm exceeds 30 dB as shown in FIG. End up. As a result, mode-competitive oscillation occurs near the wavelength of 1040 nm, which may lead to destruction of optical components such as an excitation light source.

  In order to prevent this worst situation, it is desirable to provide the optical fiber Bragg grating 60 and the reflecting film 123 as reflecting means having a reflectance of 22 dB at a wavelength of 1040 nm at both ends of the final stage YbDF 43. In particular, the reflecting film 123 as the reflecting means on the output side is highly likely to be damaged. Therefore, it is desirable to insert the reflecting film 123 as a spatial component after the beam diameter is expanded. Further, such a control method for giving an upper limit of gain by a reflecting means for reflecting light of a specific wavelength may be applied to the most upstream YbDF 41.

(Second Embodiment)
FIG. 11 is a diagram showing a configuration of main parts in the second embodiment of the optical fiber amplifying module according to the present invention. The basic configuration of the optical fiber amplification module 2 according to the second embodiment is the same as that of the optical fiber amplification module 1 according to the first embodiment described above. However, it differs from the first embodiment in that it has the first control means feedback configuration in the second embodiment. In FIG. 11, the configuration downstream from the bandpass filter 50 is the same as that of the optical fiber amplification module 1 according to the first embodiment, and is not shown.

  As described above, the gain from the input connector 11 to the optical isolator 22 is preferably about 15 dB. However, it is difficult to accurately monitor the input / output of amplified light having a low duty ratio. Therefore, as shown in FIG. 11, the optical fiber amplifying module 2 according to the second embodiment specifies the ASE light generated without the YbDF 41 disposed between the optical isolator 22 and the bandpass filter 50. An optical filter 101 that selectively reflects only the wavelength component, a light receiving element (PD) 102 that receives the specific wavelength component reflected by the optical filter 101, and a control that controls the excitation light power output from the excitation light source 81. A circuit 103 is provided. The optical filter 101, the light receiving element 102, the control circuit 103, and the excitation light source 81 constitute a first control unit.

  In the second embodiment, the control circuit 103 monitors the specific wavelength component reflected from the optical filter 101 by the light receiving element 102. When the monitor value obtained from the light receiving element 102 exceeds a preset value, the control circuit 103 performs feedback control so as to reduce the supply current of the excitation light source 81 in accordance with the excess amount. At this time, the specific wavelength component to be monitored is suitable in the vicinity of 1040 nm where the ASE light peak exists, as shown in FIG. This is because it is sensitive to a change in gain in the YbDF 41 and the absolute value of the gain is large (easy to detect).

  As described above, according to the optical fiber amplifying module 2 according to the second embodiment, the gain can be easily secured without monitoring the signal light (amplified light) having a low duty ratio, and the bandpass filter 50 is provided. Effective protection becomes possible.

(Third embodiment)
FIG. 12 is a diagram showing a configuration of main parts in the third embodiment of the optical fiber amplifying module according to the present invention. The basic configuration of the optical fiber amplification module 3 according to the third embodiment is also the same as that of the optical fiber amplification module 1 according to the first embodiment described above. However, the first control means in the third embodiment is provided with a configuration that enables protection of the excitation light source 81 without monitoring signal light (amplified light) having a low duty ratio. Different from the first and second embodiments. In FIG. 12, as in FIG. 11, the configuration downstream from the bandpass filter 50 is the same as that of the optical fiber amplification module 1 according to the first embodiment, and is not shown.

  In the optical fiber amplifying module 2 according to the third embodiment, as shown in FIG. 12, the first control means is configured by the optical fiber Bragg gratings 104 and 105 disposed at both ends of the YbDF 41, and these The optical fiber Bragg gratings 104 and 105 constitute a resonator.

  The reflectivities of the optical fiber Bragg gratings 104 and 105 are set such that oscillation starts when the gain from the input connector 11 to the optical isolator 22 exceeds about 15 dB. The center reflection wavelengths of the optical fiber Bragg gratings 104 and 105 are preferably set around 1040 nm.

  From the above description, it is apparent that the present invention can be variously modified. Such modifications cannot be construed as departing from the spirit and scope of the invention, and modifications obvious to all skilled in the art are intended to be included within the scope of the following claims.

1 is a diagram showing a configuration of a first embodiment of an optical fiber amplification module according to the present invention. FIG. FIG. 2 is a diagram showing a cross-sectional structure of a final-stage amplification optical fiber (YbDF43) in the optical fiber amplification module according to the first embodiment and a refractive index profile thereof. It is a figure for demonstrating the structure of the output collimator contained in the optical fiber amplification module which concerns on 1st Embodiment. It is the table | surface which summarized the item of each of the optical fiber for amplification (YbDF41-43) contained in the optical fiber amplification module which concerns on 1st Embodiment. It is a graph which shows the longitudinal direction distribution of the optical level in the optical fiber amplification module which concerns on 1st Embodiment. It is a graph which shows the longitudinal direction distribution of the optical level in the optical fiber amplification module which concerns on 1st Embodiment. It is a graph which shows the longitudinal direction distribution of the optical level in the optical fiber amplification module which concerns on a comparative example. It is a gain spectrum of EDF. It is an enlarged view of a gain spectrum of a first-stage amplification optical fiber (YbDF41) and a partial region thereof. It is a gain spectrum of the optical fiber for amplification (YbDF43) of the last stage. It is a figure which shows the structure of the principal part in 2nd Embodiment of the optical fiber amplification module which concerns on this invention. It is a figure which shows the structure of the principal part in 3rd Embodiment of the optical fiber amplification module which concerns on this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1, 2, 3 ... Optical fiber amplification module, 11 ... Input connector, 12 ... Output collimator, 21-23 ... Optical isolator, 31, 32 ... Optical coupler, 33 ... Polarization combiner, 41-43 ... YbDF, 50 ... Band pass filter, 60, 104, 105 ... Optical fiber Bragg grating, 70 ... Combiner, 81-84 ... Excitation light source, 101 ... Reflection filter, 102 ... Light receiving element (PD), 103 ... Control circuit, 120 ... Optical fiber, 121 ... end cap, 122 ... lens, 123 ... reflective film, 124 ... optical isolator.

Claims (9)

A seed light source that outputs pulsed light having a duty ratio of 1% or less ;
An optical fiber laser light source comprising an optical fiber amplification module,
The optical fiber amplification module is:
An input terminal for inputting the input light from the seed light source into the light propagation path;
A plurality of amplifying optical fibers arranged in a column along the light propagation path and amplifying the input light, respectively, wherein Yb element is added to the optical waveguide region ;
An output terminal for outputting the amplified light obtained by amplifying the input light from the light propagation path;
A dielectric multilayer optical filter for selectively allowing the input light to pass therethrough, of the plurality of amplification optical fibers, the first-stage amplification optical fiber through which the input light that has passed through the input end first passes And a dielectric multilayer optical filter provided between the first-stage amplification optical fiber and the second-stage amplification optical fiber through which the amplified light first passes,
An optical isolator which is provided between the first-stage amplification optical fiber and the second-stage amplification optical fiber and allows the input light to pass from the first-stage amplification optical fiber to the second-stage amplification optical fiber. And having
From the first-stage amplification optical fiber to the last-stage amplification optical fiber, the overall maximum gain is set to 50 dB or more,
First control means for setting a gain for input light in the first-stage amplification optical fiber to be equal to or less than a first predetermined value that is a value at which the dielectric multilayer optical filter is not destroyed ;
In the gain set in the first-stage amplification optical fiber, the unsaturated absorption length product of the first-stage amplification optical fiber is set to be a small signal gain,
The first control unit controls a pumping light source that supplies the pumping light so as to maintain a constant power of the pumping light supplied to the first-stage amplification optical fiber.
An optical fiber laser light source . The first stage of the unsaturated absorption fiber length product of the amplification optical fiber, the optical fiber laser light source according to claim 1, wherein a is not more than 1000 dB. A seed light source that outputs pulsed light having a duty ratio of 1% or less ;
An optical fiber laser light source comprising an optical fiber amplification module,
The optical fiber amplification module is:
An input terminal for inputting the input light from the seed light source into the light propagation path;
A plurality of amplifying optical fibers arranged in a column along the light propagation path and amplifying the input light, respectively, wherein Yb element is added to the optical waveguide region ;
An output terminal for outputting the amplified light obtained by amplifying the input light from the light propagation path;
A dielectric multilayer optical filter for selectively allowing the input light to pass therethrough, of the plurality of amplification optical fibers, the first-stage amplification optical fiber through which the input light that has passed through the input end first passes And a dielectric multilayer optical filter provided between the first-stage amplification optical fiber and the second-stage amplification optical fiber through which the amplified light first passes,
An optical isolator which is provided between the first-stage amplification optical fiber and the second-stage amplification optical fiber and allows the input light to pass from the first-stage amplification optical fiber to the second-stage amplification optical fiber. And having
From the first-stage amplification optical fiber to the last-stage amplification optical fiber, the overall maximum gain is set to 50 dB or more,
First control means for setting a gain for input light in the first-stage amplification optical fiber to be equal to or less than a first predetermined value that is a value at which the dielectric multilayer optical filter is not destroyed ;
The first control means monitors wavelength components other than the input light in the ASE light output from the first-stage amplification optical fiber, and is supplied to the first-stage amplification optical fiber according to the monitor value An optical fiber laser light source characterized by controlling the power of light . A seed light source that outputs pulsed light having a duty ratio of 1% or less ;
An optical fiber laser light source comprising an optical fiber amplification module,
The optical fiber amplification module is:
An input terminal for inputting the input light from the seed light source into the light propagation path;
A plurality of amplifying optical fibers arranged in a column along the light propagation path and amplifying the input light, respectively, wherein Yb element is added to the optical waveguide region ;
An output terminal for outputting the amplified light obtained by amplifying the input light from the light propagation path;
A dielectric multilayer optical filter for selectively allowing the input light to pass therethrough, of the plurality of amplification optical fibers, the first-stage amplification optical fiber through which the input light that has passed through the input end first passes And a dielectric multilayer optical filter provided between the first-stage amplification optical fiber and the second-stage amplification optical fiber through which the amplified light first passes,
An optical isolator which is provided between the first-stage amplification optical fiber and the second-stage amplification optical fiber and allows the input light to pass from the first-stage amplification optical fiber to the second-stage amplification optical fiber. And having
From the first-stage amplification optical fiber to the last-stage amplification optical fiber, the overall maximum gain is set to 50 dB or more,
First control means for setting a gain for input light in the first-stage amplification optical fiber to be equal to or less than a first predetermined value that is a value at which the dielectric multilayer optical filter is not destroyed ;
The first control means transmits both input light and selectively reflects a specific wavelength component around the input light wavelength, which are arranged at both the light input end and the light output end of the first-stage amplification optical fiber. An optical fiber laser light source comprising two reflecting means for Second control means for setting a gain with respect to input light in a final-stage amplification optical fiber disposed at a position closest to the output end among the plurality of amplification optical fibers to be equal to or less than a second predetermined value. optical fiber laser light source of any one of claims 1-4, characterized in that it comprises a. The second control means is disposed so as to sandwich the final-stage amplification optical fiber, and transmits two input lights while selectively reflecting a specific wavelength component around the input light wavelength. 6. The optical fiber laser light source according to claim 5, further comprising means. Of the two reflecting means, the reflecting means located on the downstream side in the propagation direction of the input light is such that the amplified light output from the final-stage amplification optical fiber propagates through the final-stage amplification light. The optical fiber laser light source according to claim 6, wherein the optical fiber laser light source is disposed in a space between a fiber and the output end. 7. The optical fiber laser light source according to claim 6, wherein the selectively reflected specific wavelength component has a shorter wavelength than any of wavelength components included in input light . The optical fiber laser light source according to any one of claims 1 to 4, wherein the optical isolator transmits light in a 1064 nm band .

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